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Union types for Kotlin

KUnion brings emulated union types to Kotlin.

Union types allow a variable to take a value of two or more types.

For example, suppose you have a data source holding resources of three different types, User, Device and Organization... the return type of a getResourceById function might be defined using Union.U3 (a union between 3 types):

fun getResourceById(id: String): Union.U3<User, Device, Organization>?
{
    // TODO call a data source
    return Union.U3.ofA(User())
}

To read the result of this function, you use the use method, which lets you specify a lambda for each type of resource that might be returned:

getResourceById("name = 'joe'")?.use(
        { user -> println("Got a user") },
        { device -> println("It is a device") },
        { org -> println("Org returned") })

Even though Kotlin infers the correct types for the argument of each lambda, you can also specify that explicitly (just to prove that the types are correct):

getResourceById("name = 'joe'")?.use(
        { user: User -> println("Got a user") },
        { device: Device -> println("It is a device") },
        { org: Organization -> println("Org returned") })

Normally, you define a typealias to avoid having to deal with Union types directly.

For example, you could define a Resource type alias in the above example:

typealias Resource = Union.U3<User, Device, Organization>

fun getResourceById2(id: String): Resource?
{
    // TODO call a data source
    return Union.U3.ofA(User())
}

Union types are often used as less general constructs such as the Either type, which can be defined as a type union like this:

typealias Either<A, B> = Union.U2<A, B>

Or the Result type (also called Try):

typealias Result<B> = Union.U2<Throwable, B>

Motivation

Union types are a very convenient way to represent the return value of functions which may produce values of different types. The classic example is the Result type, which encodes a result as either an error or a success (but never both).

For example, a function that reads a file might most certainly cause an error. As Kotlin does not have checked Exceptions, one way you could force the caller to check for an error condition would be to use the Result type:

fun readFileLines(path: String): Result<List<String>>
{
    val file = File(path)

    // turn a try/catch-based function into a union-type-based one
    return try
    {
        Result.ofB(file.readLines())
    }
    catch (e: Exception)
    {
        Result.ofA(e)
    }
}

// to use the result, we must check it first
val result = readFileLines(file)

result.use(
        { error -> println("An error has occurred: $error.") },
        { lines -> println("File $file has ${lines.size} lines.") })

It is also possible to access the value directly and check it:

println("Result is success? " + (result.asInstance().value !is Throwable))

You could have defined the Result type quite easily in Kotlin:

sealed class Result<out B>
{
    data class Success<out B>(val b: B) : Result<B>()
    data class Failure(val error: Throwable) : Result<Nothing>()
}

The problem that KUnion solves is, basically, it makes it easier to define this kind of types in Kotlin using typealias:

typealias Result<B> = Union.U2<Throwable, B>

Or ad-hoc (ie. no need to create a named type for it):

fun Int.factorial(): Int =
        if (this <= 1) 1
        else this * (this - 1).factorial()

fun describeFactorial(i: Int): Union.U2<Pair<Int, String>, Pair<Int, Int>> =
        if (i > 8) Union.U2.ofA(i to "Too large")
        else Union.U2.ofB(i to i.factorial())

(0..10).map(::describeFactorial).forEach { u ->
    u.use({ u -> println("Unfortunately, ${u.first}! is ${u.second}") },
            { u -> println("${u.first}! = ${u.second}") })
}

KUnion also adds a few useful operations to the Union types that makes it nicer to deal with their values, as you'll see in the next sections.

If you need more motivation for the concept of union types (or sum types) in general, check Chad Austin's blog post on the topic!

Usage

KUnion offers unions of 2, 3 and 4 types, called respectively, Union.U2, Union.U3 and Union.U4 (you can have more types in a Union by combining these, of course).

Each generic type of a Union is referred to as A, B, C and D, in this order. So, to create a Union.U2 whose second type is String, you can do:

val value = Union.U2.ofB("my string")

Union types provide factory functions called ofA, ofB, ofC (Union.U3 and Union.U4 only) and ofD (Union.U4 only), ie. they use the pattern ofX where X is the name of the generic type of the value.

This value would have a type of Union.U2<Nothing, String>, making it a sub-type of any Union.U2 whose second type is also a String, for example Union.U2<Int, String> and Union.U2<Boolean, String>.

Unfortunately, the order of the types matter, therefore a Union.U2<A, B> is not the same as Union.U2<B, A>, for example. If you run into this problem, use the rotate() operation, which turns the types around (see below).

Because Kotlin supports method overloading, union types are mostly useful as the return type of functions. However, having both method overloading and return types with overloading allows for the expression of powerful ideas!

Guidelines for usage

Union types can be very useful, but Kotlin already offers a few alternatives, so knowing when to use them is important.

null values are generally not welcome inside a Union. Make the Union? itself nullable if null is expected.

Below, we discuss when to use and when NOT to use union types:

When to use union types

When NOT to use union types

For example, if you need a function that may accept a String or an Int, write one version for each type:

fun overloadExample(s: String): String = s

fun overloadExample(i: Int): Int = i

Union operations

fun rotate(): Union

Rotates the types in the Union to the right.

Examples:

val a: Union.U2<String, Int> = Union.U2.ofA("a")
val b: Union.U2<Int, String> = a.rotate()

val x: Union.U3<Int, Float, Double> = Union.U3.ofC(0.3415)
val y: Union.U3<Double, Int, Float> = x.rotate()
val z: Union.U3<Float, Double, Int> = y.rotate()

fun asU2(): U2<A, B> | U2<B, C> | U3<B, C, D>>

Converts any Union to a Union.U2.

Examples:

val m: Union.U3<String, Int, Double> = Union.U3.ofC(0.3415)
val n: Union.U2<String, Union.U2<Int, Double>> = m.asU2()

fun asInstance(): Instance<*>

Returns the value of the union type's instance.

Examples:

val i1: Union.U2<String, Int> = Union.U2.ofA("hello")
val i2: Union.Instance<*> = i1.asInstance()

// notice that Kotlin can only represent the type of a Union's value as Any?
val i3: Any? = i2.value

// if you know the exact type of the Union, then the Instance will have the expected type
val i4: Union.U4.U4_1<String> = Union.U4.ofA("Hello")
val i5: String = i4.asInstance().value

fun <R> use(useA: (A) -> R, useB: (B) -> R[, useC: (C) -> R, useD: (D) -> R]): R

Uses one of the types of the Union (whatever the type of the value it holds is), returning whatever the selected function returns.

This is similar to Kotlin's when statements over sealed classes (in fact, you can use when statements with Union types, though that's not as convenient as using use()).

Examples:

val u: Union.U2<Float, Double> = Union.U2.ofA(0.5f)
val v1: Double = u.use({ f -> f.toDouble() }, { d -> d })

// or equivalently, but more verbose
val v2: Double = when(u) {
    is Union.U2.U2_1<Float> -> u.value.toDouble()
    is Union.U2.U2_2<Double> -> u.value
}

fun <E, F[, G, H]> map(mapA: (A) -> E, mapB: (B) -> F[, mapC: (C) -> G, mapD: (D) -> H]): U4<E, F[, G, H]>

Maps a Union to another Union of different type using the provided transformations.

Examples:

val p: Union.U2<Int, Float> = Union.U2.ofA(10)
val q: Union.U2<Double, Long> = p.map(Int::toDouble, Float::toLong)

equals(), hashCode() and toString()

Union types implement equals(other: Any?) such that it returns true if:

hashCode() always returns the hash code of the Union's value (or 0 for null).

toString(), in the same spirit, returns whatever the Union's value returns for toString() (or <null> for null).

The idea is to consider the Union as if it were the value itself. The Union should be considered merely a inconvenient, but necessary wrapper around a value.